In the field of medical technology, various extracorporeal blood treatment apparatuses are known which comprise an extracorporeal blood circuit. The known extracorporeal blood treatment apparatuses include for example dialysis apparatuses, which necessitate an access to the vascular system of the patient. In extracorporeal blood treatment, blood is withdrawn from the patient via an arterial hose line with an arterial puncture cannula, the blood being fed back again to the patient via a venous hose line with a venous puncture cannula. Extracorporeal blood treatment apparatuses comprise a blood pump for conveying the blood in the extracorporeal blood circuit.
Despite regular monitoring of the vascular access by hospital staff during extracorporeal blood treatment, there is in principle the risk of the venous puncture cannula slipping out of the patient's blood vessel unnoticed. Whereas slipping-out of the arterial cannula is associated with the sucking-in of air into the arterial hose line, the slipping-out of the venous cannula leads to the feared free flow of blood into the surroundings. If the slipping-out of the venous cannula is not detected immediately, therefore, there is the risk of the patient bleeding to death.
Various devices of differing design are known for the monitoring of the vascular access. The known monitoring devices generally rely on the safety devices which are present as standard in blood treatment apparatuses and which, in the event of an incorrect vascular access, trigger an immediate interruption to the blood flow in the extracorporeal blood circuit.
A monitoring device for a vascular access is described in International Patent Publication No. WO 99/29356 A1, wherein the strength of an electric current flowing through the fluid in the hose line is measured. U.S. Patent Publication No. 2004/0254513 A1 describes a monitoring device, wherein the impedance between two electrodes disposed on the arterial and venous hose line is measured. A drawback is that the known devices require the creation of an electrical connection to the fluid flowing in the hose lines.
Monitoring devices which can detect the outflow of blood at the puncture point are described in International Patent Publication No. WO 2006/008866 A1 and U.S. Patent Publication No. 2005/0038325 A1. These devices comprise a moisture sensor.
Various methods are known for determining the concentration of specific components in a patient's blood, for example for determining the concentration of haemoglobin in the blood or the haematocrit. Methods are known for measuring the concentration of blood constituents, which require the taking of a blood sample. Measuring methods are however also known, wherein the concentration of constituents in the blood flowing through a hose line is measured non-invasively. These methods are used especially when, in an extracorporeal blood treatment, the blood flows through the hose line of an extracorporeal blood circuit.
International Patent Publication No. WO 2008/000433 A1 describes a method and a device for determining the concentration of specific blood constituents in a blood-filled transparent hose line of an extracorporeal blood circuit of an extracorporeal blood treatment apparatus. The known method and the known device permit in particular the determination of the haemoglobin concentration and the proportion of red blood corpuscles (erythrocytes) in the total volume of the blood. During the measurement, the hose line is clamped between two parallel contact faces. The measurement of the haemoglobin concentration or the haematocrit is based on the scattering or transmission of electromagnetic radiation in the blood. With a light emitter, light of a specific wavelength is coupled through the transparent hose line into the blood, whereas with a light detector the scattered or transmitted light is measured. The haematocrit is then determined from the ratio of the intensity of the light entering into the blood and exiting from the blood.
In extracorporeal blood treatment methods, for example haemodialysis, haemofiltration and haemodiafiltration, an arteriovenous fistula is often applied surgically as an access to the patient's blood vessel system. The use of an implant is also possible. When mention is made below of a “fistula,” this is understood to mean any kind of connection between a vein and an artery to create a vascular access.
In the period free from dialysis, the blood flow in the fistula corresponds to a functional left/right shunt, wherein a part of the arterial blood is fed from the heart minute volume (HMV), bypassing a peripheral use, directly to the venous system and the heart. The fistula flow recirculates via the heart and lungs. The fractional part of the fistula flow in the heart minute volume is defined as the cardiopulmonary recirculation. During the dialysis treatment, the blood emitted from the left ventricle of the heart for the most part flows into the capillary systems of all the organs and to a small extent into the fistula. In the case where the blood flow in the extracorporeal blood circuit is smaller than the flow of the blood flowing into the fistula or out of the fistula, a part of the fistula blood flows through the extracorporeal blood circuit and the other part through the fistula. The extracorporeal blood, the blood flowing through the fistula and the blood coming from the capillary systems finally unite again in the return flow to the heart. If, on the other hand, the extracorporeal blood flow is greater than the fistula flow, blood from the extracorporeal blood circuit recirculates, a flow passing through the fistula from the venous to the arterial connection.
A method and a device for determining the recirculation in a fistula or the cardiopulmonary recirculation are described in International Patent Publication No. WO 2009/065611 A1. The known method and the known device are based on the fact that the sum of the fistula recirculation (RA) and the cardiopulmonary recirculation part (RCP), i.e., the total recirculation (R), is determined for two different blood flow rates. International Patent Publication No. WO 2009/065611 A1 also describes the theoretical background to the effect of the fistula recirculation and cardiopulmonary recirculation. These effects are also described in the technical article “Automatic Measurement of Recirculation” by Krämer and Polaschegg, EDTNA-ERCA Journal, Vol. XIX, No. 3 (1993).